# Persistence and failure recovery

The project format remains v4. Readers migrate v1/v2/v3, validate bounded input
and atomically install a candidate only after complete success. Existing tests
cover exact migration/roundtrip, truncations, malformed references, counts,
IDs, nonfinite values and failed GUI/backend installation. Native processors
are session-only; saving their graph returns UNSUPPORTED rather than writing
process addresses or an unreadable project. See PROJECT_FORMAT.md.

Project Save writes a mode-0600 same-directory `NAME.tmp.XXXXXX`, flushes and
fsyncs it, closes and renames it over the destination. Before rename, handled
failures remove the temporary and retain the destination. POSIX rename gives
complete old-or-new destination contents during process failure. The directory
is not fsynced, so this is not a power-loss durability guarantee. Symlinks are
replaced rather than followed at rename; original file permissions are not kept.

Recording similarly writes/finalizes a same-directory temporary, then renames
it. Stop must synchronize callbacks before draining/finalizing. Ring overflow
returns an explicit error and may finalize a partial take for manual recovery;
I/O failure retains the previous destination. A killed recorder does not finish
its WAV header. Source WAV files remain external absolute paths; Save As does
not collect them. Back up the project and assets together.

## After a crash

1. Preserve the destination and any `.tmp.*` files before attempting repair.
2. Run `xtc-check --validate project.xtc` for metadata validation without a GUI,
   display or audio server. This does not verify assets or realtime preparation.
3. A valid destination can be opened normally. If only a temporary contains useful
   data, copy it to a separate candidate and validate that copy before replacing
   anything. Partial project files are rejected; there is no salvage parser.
4. An unfinished WAV temporary may have a zero/incorrect data length and needs
   a separate recovery tool or manual header repair on a copy. xtc does not guess
   its intended length or automatically rename orphan temporaries.
5. Missing assets preserve silent clips. Use Relink selected audio, then Play to
   revalidate. For server loss/format changes, Stop/Reconnect and explicit routing
   are required. Panic/F9 clears held notes. Failed close retains ownership for retry.

There is no autosave, crash journal, automatic orphan recovery or persisted Undo
history. Existing finalized files are protected against the tested process-level
failures; unsaved edits/in-flight takes can still be lost.

`make test-recovery` uses temporary directories and child processes only. It
forces an EFBIG-style write failure with a child-local file-size limit, kills
eight project writers while temporaries are visible, and kills a recorder after
draining but before finalization. Complete old/new destinations and explicit
orphan temporaries are verified. It does not cut power, terminate the shared
audio server, simulate a failing filesystem, or qualify disk/device endurance.

## Phase 13 retained recording prefixes

Recording now defaults to no-clobber publication; the GUI requires an unused
path. Failed nonempty temporary recordings survive destruction. Query the
recorder recovery path before destruction, or use the GUI's reported path.
`xtc-check --recover-recording TEMP NEW.wav` reconstructs a canonical mono
float32 WAV from complete on-disk frames, ignoring a trailing partial frame.
It never changes the source or overwrites an existing destination. This
supersedes the earlier lack of a WAV recovery tool described above. See
AUDIO_FILES.md for format/ownership/size limits and `make test-recording`.

## Plugin load and replacement failures (Phase 25)

GUI Open reports an unavailable/incompatible plugin's label and absolute module
path and retains the current document. Preserve the saved file; restore a
compatible module at its original path and retry Open. `xtc-check --validate`
checks metadata without executing plugins, so success does not certify that a
module exists or initializes. Metadata-only C load/save preserves unavailable
plugin records. There is no automatic missing-plugin substitution or GUI relink
for an unopened document.

For an already loaded project, Replace selected plugin preserves wires and node
ID only for the same audio layout. New defaults replace controls/automation;
Undo restores the original configuration. Failed discovery/compile/instantiate
retains the original graph, controls and history position. Cancel in the generic
parameter editor discards its temporary values. Reset defaults clears automation
only when applied. No host recovery can contain a crash or hang in trusted native
plugin code; LADSPA has no runtime error-return channel.